1. Field of the Invention
The present invention relates to an image heating device suitably used as a fixing device included in an image forming apparatus employing an electrophotography recording technique.
2. Description of the Related Art
A fixing device included in an image forming apparatus employing an electrophotography recording technique generally includes a heater. The heater is connected to an AC power source through a switching element such as a triac, and electric power is supplied from the AC power source so that the heater generates heat.
When the heater (a heating element) is controlled, a harmonic current and a flicker are required to be suppressed. Japanese Patent Laid-Open Nos. 2005-195640 and 2011-95314 disclose methods for suppressing generation of a harmonic current and a flicker in a fixing device including a plurality of heaters. Specifically, a waveform in which a first half cycle in which current is supplied or not supplied in entire half cycles of alternate current and a second half cycle in which current is supplied in portions of half cycles of the alternate current are mixed is applied to two heaters which may be independently driven. Furthermore, the alternate current supplied to one of the heaters and the alternate current supplied to the other of the heaters are controlled such that the first and second half cycles overlap with each other in half cycles of the same phases of the alternate current supplied to the two heaters. This control is advantageous since synthesized current supplied to the two heaters becomes similar to a sine wave, and therefore, a power factor is improved.
In some fixing devices including a plurality of heaters, for example, a fixing device disclosed in Japanese Patent No. 4208772, a power supply ratio between heaters is changed in accordance with a size of a recording material and different heat generation distributions are set.
Here, when the control methods disclosed in Japanese Patent Laid-open Nos. 2005-195640 and 2011-95314 are employed in an apparatus which changes a power supply ratio between heaters in accordance with a size of a recording material, such as the apparatus disclosed in Japanese Patent No. 4208772, the following problem arises. That is, when a recording material of a small size is subjected to a fixing process, electric power to be supplied to one of heaters is required to be reduced in order to suppress a heat generation amount in a region in which the recording material does not pass. However, it is found that electric power is concentrated on certain half cycles of an AC waveform and a flicker is further generated.